This became a bigger deal with COVID, because crashing O2 levels could occur even when someone is subjectively feeling relatively fine - e.g., they think they've got mild COVID and then are all of a sudden at deaths door with little warning. Not something most people generally need though.
For healthy people in normal conditions it's rather pointless. Continuous monitoring can be useful for patients with known or suspected pulmonary conditions, such as emphysema, COVID-19, or pneumonia. Monitoring during sleep can be useful for diagnosing sleep apnea. There is also an edge case for high altitude mountaineering and flying in unpressurized aircraft where hypoxia can be an issue.